Liquid crystal display devices are used in various products, including clocks, calculators, household electrical appliances, measuring instruments, automotive instrument panels, word processors, electronic organizers, printers, computers, and televisions. Typical types of liquid crystal display devices include twisted nematic (TN), super-twisted nematic (STN), dynamic scattering (DS), guest-host (GH), in-plane switching (IPS), fringe-field switching (FFS), optically compensated birefringence (OCB), electrically controlled birefringence (ECB), vertically aligned (VA), color super-homeotropic (CSH), and ferroelectric liquid crystal (FLC) display devices. Although conventional liquid crystal display devices are statically driven, multiplexed liquid crystal display devices have been commonly used. Among the mainstream schemes are passive-matrix driving and, more recently, active-matrix (AM) driving with elements such as thin-film transistors (TFTs) and thin-film diodes (TFDs).
Silicon-based semiconductors are known for use in thin-film transistors for active-matrix driving. Recently, thin-film transistors fabricated from oxide semiconductors, such as zinc oxide and In—Ga—Zn—O, have also attracted attention for use in liquid crystal display devices (see PTL 1). Oxide semiconductor thin-film transistors have higher field-effect mobilities than silicon-based thin-film transistors and thus allow for improved display device performance and reduced power consumption. Accordingly, liquid crystal device manufacturers are focusing their efforts on the development of oxide semiconductor thin-film transistors, including the use of arrays thereof.
Unfortunately, oxide semiconductor thin-film transistors have low reliability due to variations in electrical characteristics. The variations in electrical characteristics are attributable to lattice defects, such as oxygen defects, which occur when oxygen desorbs from an oxide semiconductor layer. As a solution to this problem, a method has been researched that involves controlling the oxygen atmosphere conditions during the deposition of an oxide semiconductor to reduce the electron carrier concentration so that fewer oxygen defects occur (see PTL 2).
A liquid crystal composition used for a liquid crystal layer of a liquid crystal display device is subjected to strict impurity control since impurities present in the composition greatly affect the electrical characteristics of the display device. It is also known that impurities remaining in the material used for alignment layers, which directly contact the liquid crystal layer, migrate into the liquid crystal layer and affect the electrical characteristics thereof. Accordingly, research has been conducted on the influence of impurities in alignment layer materials on the characteristics of liquid crystal display devices.
Although research has been conducted on various solutions to the problem of lattice defects such as oxygen defects, as discussed in PTL 2, they have been unsuccessful in sufficiently reducing the desorption of oxygen from an oxide semiconductor layer. As oxygen desorbs from an oxide semiconductor layer, it diffuses into and alters an insulating layer covering the oxide semiconductor layer. A typical liquid crystal display device includes only a thin insulating layer, or a thin insulating layer and a thin alignment layer, between oxide semiconductor layers of thin-film transistors and a liquid crystal layer to separate the liquid crystal composition from the oxide semiconductor layer; therefore, the diffusion of oxygen desorbed from the oxide semiconductor layer and the resulting alteration of the insulating layer result in insufficient separation of the liquid crystal layer from the oxide semiconductor layer. As a result, the oxygen desorbed from the oxide semiconductor layer will affect the liquid crystal layer. The diffusion of impurities such as oxygen desorbed from the oxide semiconductor layer into the liquid crystal layer may decrease the voltage holding ratio (VHR) and increase the ion density (ID) of the liquid crystal layer and may thus cause display defects such as white spots, uneven alignment, and image-sticking.
However, as disclosed in PTL 2, the previous inventions are intended to reduce the desorption of oxygen from oxide semiconductors; no research has been conducted on the direct relationship between oxide semiconductor thin-film transistors and liquid crystal compositions.